Retroreflective sheeting is used in highway safety applications to reflect light from a vehicle's headlights back to the eyes of the driver. Retroreflective sheeting typically comprises a transparent layer having a front light-receiving surface and a rear surface with an array of retroreflective elements formed thereon. When incident light reaches the front light-receiving surface, it passes therethrough, impinges on the retroreflective elements, and reflects back out through the front surface at a two-dimensional retroreflective pattern. The divergence range of a retroreflective array is the range of angles for which this pattern is relatively intense so as to be useful for its intended purpose. It is usually preferable for retroreflective sheeting to produce a pattern of retroreflected light with sufficient intensity over a divergence range of at least 1° and preferably 3° or more.
An intentional aberration in one or more retroreflective elements can positively affect (e.g., broaden without introducing undesirable diffraction) the divergence range of an overall array. Historically, aberrations in retroreflective arrays have stemmed from the geometries introduced into the master plate during ruling, these ruled geometries then being transferred to copies/clusters and eventually to the production tool which is used to form the retroreflective sheeting. (See e.g., U.S. Pat. Nos. 3,712,706, 4,775,219, 4,938,563, 6,015,214, US2003/007815A1, etc.). U.S. Pat. No. 6,871,966 each discloses a method of introducing aberration-producing geometries by controlled working of the second “non-arrayed” surface of the master substrate after tooling or by similar controlled working of copy substrates made from the master substrate. (This patent is assigned to the assignee of the present invention and its entire disclosure is hereby incorporated by reference.)